European Patent No. 705 177 describes a method and a device for controlling a drive unit, in which the drive torque is measured, which is the maximum that can be transferred from the drive to the street. In this context, the drive torque is, calculated by taking into account the air resistance, the rotational resistance torque, the acceleration resistance torque, etc., and from this, a drive torque is determined for the drive unit of the vehicle, corresponding to the maximum transferable drive torque under the prevailing conditions. If instability should occur in at least one drive wheel, i.e., if this drive wheel shows a tendency to spin, the torque of the drive unit is reduced to the drive torque that is calculated to be the maximum possible.
When starting up on roadways having a lower coefficient of friction, e.g., on snow or ice, the drive wheels, due to their high drive torque (caused by a powerful excess torque in first gear), very frequently begin to slip significantly. The known solution counteracts this increased slipping by engaging the engine and, optionally, the brakes, as described, it being possible in these situations, as a result of the slowly accumulating acceleration of the vehicle connected with high slipping at a relatively low drive torque, that the reduction of the drive torque can turn out to be so pronounced that the slipping is completely eliminated. If the drive wheels take on the velocity of the reference wheels, then the acceleration of the motor vehicle declines. Associated with this is a pitching of the vehicle, which is felt by the driver subjectively to be extremely uncomfortable and intrusive.
The same applies, outside the start-up range, in which in some situations on roadways having a lower coefficient of friction, a powerful reduction of the drive torque takes place as a result of the engagement of the anti-spin regulator. This is especially the case when the drive wheels have significant slipping in response to a change in the coefficient of friction of the roadway (e.g., in the transition from snow to an ice plate). Since, apart from the start-up range, in a situation of significant slipping, a stability-endangering state cannot be excluded, in particular during cornering, the torque reduction takes place very rapidly, resulting frequently that, here too, the velocities of the drive wheels are reduced to very close to the reference velocity. A corresponding pitching of the motor vehicle is the consequence.
A different problem area is represented by the so-called xe2x80x9cpower startxe2x80x9d (starting up with squealing, spinning drive wheels) on roadways having a high coefficient of friction. This situation can also arise in a gradual start-up, for example on wet roadways. Here too, the drive wheels can experience significant slipping. An engagement of the anti-spin regulation, here too, causes a very significant acceleration slump, which is also felt by the driver to be very unpleasant.
It is an object of the present invention to improve the control of a drive unit such that the disadvantages described do not arise or are reduced, without significantly impairing the stability or the traction of the motor vehicle.
German Patent No. 44 30 108 describes a procedure in which the coefficient of friction of a roadway is estimated on the basis of the drive torque of the vehicle and the wheel slip of at least one drive wheel.
As a result of the procedure described below, the aforementioned disadvantages are effectively eliminated or reduced. This is due to the fact that by stipulating a minimal drive torque for engaging the anti-spin regulation, an excessive, uncomfortable reduction of the drive torque is avoided and the control comfort of the anti-spin regulator is significantly increased, while maintaining stability and traction.
It is particularly advantageous that, as a result of the use of the procedure described below, a uniform start-up is made possible on roadways having a lower coefficient of friction, the drive wheels rotating in state of continual slipping. This has particular advantages in vehicles having fewer cylinders, in which the torque reduction is realized by limiting the fuel injection pulses. The procedure described below effectively avoids an uncomfortable process, the so-called engine shaking, in response to the engaging of the anti-spin regulation.
In an advantageous manner, the start-up process of the motor vehicle is improved, since, despite the spin detected by the anti-spin regulator, the value for the torque cannot sink below the minimum reducing torque due to the drive torque reduction, so that the start-up in a continuous slip can take place without the motor vehicle pitching. In an advantageous manner, the regulation can be adjusted, apart from the start-up range, such that maximum stability is achieved with the greatest possible comfort.
In a particularly advantageous manner, the level of the pre-determined minimum torque is a function of the driving situation.
Advantages as a result of the procedure described below are also yielded in regard to roadways having a high coefficient of friction or wet roadways, in which an elevated torque is required in order to carry out a so-called xe2x80x9cpower start.xe2x80x9d The acceleration slump caused by the engagement of the anti-spin regulation is reduced, or ideally prevented.